A few code lists that FHIR defines are hierarchical - each code is assigned a level. For value sets, levels are mostly used to organize codes for user convenience, but may follow
code system hierarchy - see Code System for further information

Source

The source of the definition of the code (when the value set draws in codes defined elsewhere)

Code

The code (used as the code in the resource instance). If the code is in italics, this indicates that the code is not selectable ('Abstract')

Display

The display (used in the display element of a Coding). If there is no display, implementers should not simply display the code, but map the concept into their application